The present invention relates to a thermal type flow sensor equipped with a sensor element having an exothermic resistor formed on a substrate as a detecting member for detecting the flow rate of a fluid such as air passing through an air intake passage of vehicle-mounting internal combustion engine. In particular, the present invention relates to a thermal type flow sensor which is designed to exhibit improved accuracy in the measurement of flow rate.
As for the flow sensor for detecting the intake air flow of an internal combustion engine to be mounted on a vehicle, a thermal type air flow sensor which is capable of directly measuring the mass flow rate is mainly employed.
In recent years, there has been proposed an idea to manufacture the sensor element of thermal type flow sensor on a semiconductor substrate such as a silicon substrate by making use of micromachine techniques. In the thermal type flow sensor of this semiconductor type, an exothermic resistor and a temperature-sensing resistor are formed on a thin film portion having a thickness of several microns formed on a semiconductor substrate. Since this sensor is formed of a thin film, the heat capacity thereof is small, thereby making it possible to realize high-speed response and low power driving. Further, it is possible, through fine working, to easily fabricate a structure which is capable of detecting a difference in temperature between the upstream side and the down stream side of the exothermic resistor to thereby determine if the flow is forward flow or backward flow.
Since the thermal type flow sensor of semiconductor type is formed of a plate-like sensor element, the accuracy of measurement will be greatly affected when any disturbance is caused to generate in the flow of fluid around the sensor element. Therefore, the problem is how to create the structure that cannot be badly affected by the disturbance in flow of fluid as described above.
As for the prior art which aims at overcoming the aforementioned problem, there is known a thermal type flow sensor which is described in JP Patent Application H10-300546 (1998). The thermal type flow sensor shown in this publication is designed so as to minimize the error of measurement that may be caused to occur due to the disturbance of flow at the edge portion of the base portion for mounting the sensor element or at the edge portion of the substrate of sensor itself.
In most of the thermal type flow sensor of semiconductor type, the sensor element is mounted on the inner wall of passage for measurement or measuring passage (base portion) where a fluid is enabled to flow through or on the inner wall of sub-passage (the inner wall of measuring passage) which is provided in a main passage. As for the specific state in the mounting of the sensor element, a rectangular recessed portion is formed at the base portion and the sensor element is fitted inside the recessed portion and adhered thereto by making use of an adhesive and the like.
On the occasion of mounting the sensor element inside the recessed portion, the surface (detecting portion) of sensor element should preferably be made flush with the surface of the inner wall of measuring passage. Depending on the manufacturing conditions however, the surface of sensor element may be protruded or depressed from the surface of the inner wall of measuring passage (forming a step portion), thus creating non-uniform surface.
For example, when the sensor element is protruded from the surface of inner wall of measuring passage, the disturbance of flow (peeling) is caused to generate at the edge portion of sensor element. As a result, true fluid flow may be caused to differ from the fluid flow around the sensor element, thus generating an error of measurement. In order to minimize this measurement error, the thermal type flow sensor described in the aforementioned patent publication is constructed such that the exothermic resistor of the sensor element is disposed away from the edge portion of sensor element where the peeling thereof may be generated. However, in order to secure a sufficient isolation distance from the edge portion of sensor element, the surface area of sensor element is required to be increased, inevitably leading to an increase in manufacturing cost.
Furthermore, when the sensor element is protruded from or made flush with the surface of inner wall of measuring passage, dusts that have been intermingled with the fluid may impinge against the thin film portion of sensor element, thus raising a problem that the sensor element is caused to fracture.
Further, when the sensor element is depressed from the surface of inner wall of measuring passage, the disturbance of flow (peeling) may be caused to occur due to the step portion between the surface of inner wall of measuring passage and the surface of sensor element.
Whereas, in the case of the thermal type flow sensor described in the aforementioned JP patent publication, the aforementioned problem of non-uniformity on the occasion of mounting the sensor element is not taken into account. Even if a step portion is caused to generate due to the protrusion or depression of the surface of sensor element from the surface of inner wall of measuring passage, the problem to be raised by the step portion may be overcome by securing a sufficient distance between a peeling portion at the step portion and the exothermic resistor. However, this countermeasure would lead to substantial increase in area of the sensor element as described above, resulting in increase of manufacturing cost.